Interferometric analysis provides a nondestructive method of determining the presence or absence of anomalies in an object. Interferometric apparatus typically use holography or shearography techniques to compare images of an object taken both before and after an applied stress. Essentially, this apparatus analyzes the deformation associated with the applied stress to detect irregularities in deformation associated with anomalies within the object. A typical use for interferometric analysis occurs in the production of vehicle tires to inspect for internal defects.
Generally, a tire is placed inside a vacuum chamber and an image of one section of the surface of the tire is recorded before the application of the vacuum. After the initial recording, the tire is stressed by the application of a vacuum inside the chamber, and an image of the surface of the tire is again recorded. By superimposing the pair of images for deviations in the pattern of deformation, internal anomalies can be detected.
More specifically, in the technique of holographic interferometry, an object, such as a tire, is illuminated with coherent light, and an image of the interference pattern of reflected light from the object's surface is recorded. The object is thereafter stressed, such as by a change of temperature or pressure, and a second interference pattern is recorded in similar fashion. A hologram, developed from the pair of recorded images, is illuminated with the appropriate reconstructing light beam. This process produces an image of the illuminated object which includes superimposed fringe lines resulting from the interference between the two previously created interference patterns. These fringe lines represent contours of equal displacement of the object's surface, revealing the existence of undesirable strains in the object.
The technique of shearographic interferometry creates an interferogram by producing a pair of images of the object, shifted relative to one another, through an optical system such as a lens and wedge array. The pair of overlapping images produced by the lens and wedge array form an interferogram. When two interferograms formed of an object in two states of stress are caused to interfere with one another, the resulting interference pattern contains fringes corresponding to contours of constant strain.
Both types of interferometric analysis are discussed in the inventor's prior U.S. Pat. Nos.; 4,139,302 for a method and apparatus for interferometric deformation analysis, and 4,690,552 for an optical method and apparatus for strain analysis. These patents provide further background on the different techniques of interferometric analysis.
In any type of interferometric apparatus, an interference pattern must detect minute deviations during object deformation. The coherent light source power and beam ratios, vacuum levels, exposure variables, and the associated apparatus placement angles must be calibrated for optimum operation. Proper calibration of all these factors must be maintained to correctly detect different types of anomalies in an object, at various positions and depths.
While recording an image, an improperly calibrated interferometric device may not detect anomalies in a particular area of view, or throughout the entire area of view. Further, the device may appear to detect a nonexistent anomaly or an anomaly of incorrect proportion. Unfortunately, there is no nondestructive method to determine whether the device has correctly identified all anomalies in a given object.
Consequently, a need exists for determining whether individual interferometric devices are properly calibrated, and, if not properly calibrated, the specific area or areas of view where recalibration is necessary.